Atmospheric pressure plasma jet (APPJ) falls into one of the most promising non-equilibrium low temperature plasma sources which are convenient for multiple applications. In order to achieve the best possible results in applications and explain the mechanisms that lead to the modification of the samples it is necessary to perform a detailed diagnostics of plasma source. Many studies showed that the low- frequency plasma jet's plume is made of fast pulsed atmospheric pressure plasma streamers (PAPS). In this study we show that the change in the concentration of water vapour within the tube, where the feeding gas flows, significantly affect the formation of PAPS.